Hey Jim, I'm sure by "fiberglass resin" your mean polyester resin. It's okay for laminating "new" parts, but not so good for repair work.
In my experience epoxy laminating resin and 1.5oz. fiberglass cloth are the repair materials of choice on old fiberglass (hand laminated or bulk molded (BMW)). Epoxy adheres much better than polyester/
But you have to take some pain to make sure that the base material is free of dirt and grease, and wax*, and it's important that you rough up the surface around the crack. It will also help if you can groove out the crack into a bit of a "V". When you get ready to make the repair take a bit of the cloth and chop it up into fine pieces (1/4" long or so). * The interior surfaces of the fairing may have a residual amount mold release wax on them.
Mix up the amount of resin you are going to use for the entire repair and then use enough of it, with the chopped up material, to make a putty - smear the putty into the "V" and then continue with the rest of the cloth material.